Recently, flat-panel displays have achieved increasing acceptance in the marketplace, particularly with regard to small, hand-held television sets and portable computers. These displays are quite thin compared to cathode ray tubes, yet are able to display video and other information. Various attempts have also been made to develop large flat-panel displays as replacements for conventional cathode ray tube display systems, particularly in situations where a great deal of information must be processed and displayed, such as in proposed high definition television (HDTV) systems.
Although the primary use of flat-panel displays are for television and computer displays, flat-panel displays can be used in any number of different applications, such as for highway signs, office building directories, graphics presentations, and in avionics.
An example of a flat-panel display is shown in Brody U.S. Pat. No. 4,980,774, issued Dec. 25, 1990, and assigned to the assignee of the present application. The Brody '774 patent describes a modular flat-screen, direct-view display system that utilizes diffuse light for backlighting, where the displayed image is uninterrupted by the boundaries between modules.
Currently, a number of different materials and techniques are utilized in emissive and non-emissive flat-panel displays. For example, some flat-panel displays use liquid crystals as the display medium, while others use a gas-discharge plasma or electroluminescent material.
Recently, polymer dispersed liquid crystal (PDLC) material has been applied to flat-panel display systems. A useful form of PDLC material for display applications is a thin layer (typically 20 .mu.m thick) of a polymer in which micron-sized liquid crystal droplets are embedded. The thin layer of PDLC material is usually sandwiched between two sheets of glass having conductive coatings facing the PDLC layer. These conductive coatings act as electrodes to apply an electric field to the PDLC material. Depending on the applied field, the PDLC material either scatters the light incident to it or is transparent. Also, the degree of scattering can be electrically controlled.
In the relaxed (non-energized) state, the birefringent nematic droplets of the PDLC layer strongly scatter light because the random orientation of the liquid crystal droplets causes a mismatch between the refractive indices of the droplets and the surrounding polymer binder. In the energized state, when applied voltage on the electrodes creates an electric field, the liquid crystal molecules of the PDLC material align themselves with the electric field. In this condition, the refractive indices between the droplets and the surrounding polymer are nearly matched for light entering the PDLC layer perpendicularly. Therefore, light entering the PDLC layer perpendicularly passes through the layer virtually undisturbed. Upon removal of the voltage, the liquid crystal droplets return to their original random orientation. The voltage applied to the electrodes can be varied in order to adjust the degree of scattering of the droplets.
Another important feature of PDLC material is that the liquid crystal droplets are encapsulated in a polymer that acts like a solid. Therefore, the display medium may be brought out to the very edge of individual display modules, thus permitting minimal spacing between individual picture elements or pixels of adjacent modules and allowing for an almost seamless appearance to the entire display. In this regard, see, Brody U.S. Pat. No. 4,980,774.
There are various well known ways of controlling particular picture elements in a display such that each picture element is in an "off" or an "on" state. One way is direct addressing where each picture element is directly connected to the driver electronics for 100% of the time. An example of direct addressing is a segmented display, such as in some LCD watches, where each segment of the alphanumeric character (for example, the top horizontal member in the number "5") is an independent electrode. However, as the amount of information that must be displayed increases, a greater number of electrical connections must be used in a segmented display, thus rendering such displays impractical.
Another means for addressing the electrodes is the use of matrix techniques. In matrixing schemes, a particular picture element or pixel is activated by the driver electronics for only a fraction of the time. However, each picture element has sufficient "memory" and/or is "refreshed" often enough such that there is no discontinuity in the viewed display.
In the context of a flat-panel display, there are two matrixing techniques that are commonly used, namely, multiplexing and active matrix. A multiplexed display consists of two substrates having transparent conductive stripes at right angles to one another. The area defined by the overlap of the conductive stripes represents individual picture elements or pixels of the display. A layer of liquid crystal or other appropriate display medium is located between the electrodes. As is well known in the art, the timing and relative amplitudes of the electrical signals applied to the conductive stripes allow for different voltages to be applied to different pixels in the display, thus resulting in some pixels being "on" and others being "off". However, as the number of vertical or horizontal lines on a display increase, the number of conductive stripes on the appropriate substrate must be increased. This reduces the time available to address each individual pixel, resulting in a decrease in the contrast that can be achieved between "on" and "off" pixels. A practical upper limit for such displays is in the order of a few hundreds of lines (stripes).
In active-matrix displays, each picture element or pixel on the display contains a thin film transistor (TFT) or other electronic switch that sets the state of that pixel when voltage on a data line is allowed to pass onto the pixel electrode. By this technique the brightness of each picture element of a display can be independently controlled. See, for example, U.S. Pat. Nos. 3,840,695 (Fisher) and 4,980,774 (Brody).
One particular type of a flat-panel display is a direct-view, backlit flat-panel display. This kind of display modulates light emanating from an area source of diffuse light. This source of diffuse light is usually very shallow, for example, a fluorescent lamp or a plurality of fluorescent lamps, resulting in a shallow depth for the entire display.
Twisted nematic liquid crystals, addressed either by multiplexing or active-matrix techniques, are commonly employed as the element for modulating the light in direct-view flat-panel display systems. The light from the diffuse light source is modulated by the liquid crystal material in combination with polarizing elements and reaches the viewer directly without the need for any additional optical elements or a projection screen--thus the reference to such a system as a "direct-view" system.
The performance of these direct-view displays has been steadily improving through advances in the chemistry of liquid crystals and addressing techniques. However, the maximum viewing area of active-matrix, direct-view displays is limited by the size of available active-matrix substrates. A technique for making a large active-matrix direct-view display by assembling an array of display modules is described in Brody U.S. Pat. No. 4,980,774. The necessity to conceal the seams between modules in such a direct-view system (in order to create a seamless image) makes it difficult to use standard liquid crystal materials that require a seal at the periphery of each module. Instead, PDLC material is used in place of a standard liquid crystal material since PDLC material can be brought to the very edge of each module without the need for a seal.
The inherent disadvantage of using PDLC material as the light modulating medium in a direct-view display stems from the fact that a diffuse light source is modulated by a scattering medium. For example in the case of an ideal source of diffuse light (Lambertian surface) adjacent to an ideal scattering medium, there is no intensity modulation, in other words, the contrast ratio is 1:1. (The contrast ratio is the ratio of the light intensity for a particular pixel of the display in an "on" condition compared to the light intensity for that pixel in the "off" condition.) The scattering medium redistributes the light rays, but does not affect the intensity of the light reaching the viewer.
To produce an acceptable contrast ratio with PDLC material in a direct-view display device, a dichroic dye is incorporated in the liquid crystal droplets of the PDLC material. The elongated molecules of the dye align themselves with the molecules of the liquid crystal. In the energized state, most of the incident light passes through the PDLC layer in a straight line and is attenuated only by the dyed liquid crystal droplets encountered along this single direct path. In addition, the aligned dye molecules present a smaller "cross-section" for those rays that are essentially oriented in a perpendicular fashion. In effect, the dye slightly reduces the transmittance of the PDLC layer.
In the scattering state, the light rays undergo several refractions inside the PDLC layer before they exit either in a forward or backward direction. Since the liquid crystal droplets are randomly oriented, the "cross-section" of the dye molecules is (on the average) higher, and significant attenuation of the transmitted light results.
In practice, the highest achievable contrast ratio for a direct-view display using a dyed PDLC layer is limited by the requirement for an acceptable value of transmittance. Further, any electronics mounted in the display between the light source and the display surface may cast a shadow. Techniques employed to suppress or eliminate the shadowing further limit the achievable transmittance of the display system.
An alternative to a direct-view system is a projection system where the information (image) is illuminated with a bright light source and is projected through a lens or system of lenses onto a screen. This lens or system of lenses is known as the "objective". During the projection process, the image is magnified, depending upon the image size desired. The image can be projected on a screen that is in front of the projector and the viewer whereby the image is reflected back to the viewer. This is referred to as front projection. Alternatively, the screen can be placed between the viewer and the projector, whereby the screen is illuminated with the projected image from behind. The image is then scattered forward towards the viewer. This is called rear projection.
One commonly known use of a rear-projection device is projection TV. The images, which are formed on a faceplate of one or more specially designed CRT's, are projected onto the rear-projection screen from within the CRT enclosure, while the viewer is observing the images in front of the TV set on the rear projection screen. This technique has recently been demonstrated for displaying high density, high resolution images in HDTV systems.
The major advantage of projection displays is the ease with which a large area display can be achieved. High quality images several feet in diagonal are commonly projected by commercially available products. However, the physical depth of such a projector is determined by the required optical path, which is also typically several feet. Even if the optical path is folded by the use of mirrors, it still results in a rather bulky and heavy device.
A variation of a projection system utilizes what is called a light valve. A light valve is a thin device that can control (modulate) the intensity of light passing perpendicularly through it in accordance with a signal applied to the components that control the light valve. When a light valve is inserted into the path of projected light, images formed on the light valve surface are projected with the desired magnification toward a screen. A small twisted nematic liquid crystal display (LCD) has recently been utilized as the light valve of a front-projection commercial television system made by Sharp Corporation.
A special category within the family of LCD based projection systems is the light valve technique utilizing the phenomenon of light scattering. The modulation of the light is achieved by scattering some of the light into a much wider solid angle than the solid angle subtended by the input aperture of the objective lens. Thus, by varying the degree of scattering, the intensity of light reaching the viewer is controlled. A projection optical system based on scattering in this manner is a type of what is referred to as a Schlieren optical system. (Although other techniques, such as light diffraction or refraction, have been employed in Schlieren optical systems, for purposes of convenience and clarity in this application, only the scattering phenomenon in Schlieren optical systems is discussed herein.)
In a standard Schlieren optical system, when the light valve is in its non-scattering state, the light passes through the light valve without disturbance and is then obstructed by an opaque stop, known as a Schlieren stop. No light reaches the screen. When a voltage, or some other control means, is applied to the light valve, the light valve scatters the light and the scattered light bypasses the stop and is then collected by a projection lens and imaged on a screen.
A Schlieren optical system can be rearranged into what I call a reversed Schlieren optical system such that 1) light reaches the screen when the light valve is in its non-scattering state, and 2) light is mostly absorbed by some structure when the light valve is in its scattering state.
Although the use of standard Schlieren optical systems and reversed Schlieren optical systems have been known for some time, the only projection systems known to the applicant that have been made using the Schlieren concept have projected images several feet away to achieve the required size of the projected image.
One embodiment of the microprojection, flat-panel display system apparatus and method of the present invention is designed to rearrange the optics of reversed Schlieren-type projectors so as to result in a large flat-panel display that overcomes many of the problems of existing flat-panel systems. An alternate embodiment of the apparatus and method of the present invention utilizes a standard Schlieren-type arrangement.
In order to obtain optimum performance of the microprojection display system of the present invention, a thin illuminator that produces collimated light is desirable. Existing commercially available illuminators that produce collimated light generally have considerable depth relative to the overall desired thickness of the display system. Therefore, the present invention is also directed to a thin fiber-optic illuminator of minimal depth that can produce collimated light. The illuminator of the present invention, although primarily intended for use with non-emissive, flat-panel display systems, can also be used in any application where large-area collimated radiation is required.
Further, with regard to existing direct-view, flat-panel display systems, there remains a need for improved thin illuminators that provide an area source of diffuse light. Certain variations of the thin fiber-optic illuminator of the present invention can be used to produce such diffuse light.